When I was shooting small .45 & .54 caliber guns, the limiting factor was always recoil. I never came anywhere near what could be considered a 'standard' load. These small guns could not be fired unrestrained and even with them tethered, light loads would put on a real show.
That was a LONG time ago, and I can't remember what the actual measurements were. But I do recall that I was using a powder flask designed for C&B revolvers to measure out the charges. I used the .36 caliber spout to fire the .45, and the .44 spout to fire the .54. I was using patched round lead balls.
Even this underload required a shooting platform with tethers. But despite all that, the little guns were quite accurate once I became familiar with their POAs. Especially the .45, which was rifled.
(edited because of my faulty memory)